A MULTIREFERENCE CONFIGURATION-INTERACTION STUDY OF THE HYPERFINE-STRUCTURE OF THE MOLECULES CCO, CNN, AND NCN IN THEIR TRIPLET GROUND-STATES

The hyperfine structures of the isoelectronic molecules CCO, CNN, and NCN in their tripler ground states (X(3) Sigma(-)) are investigated by means of ab initio methods. The infrared frequencies and geometries a re determined and compared with experiment. Configuration selected mul tireference configuration interaction calculations in combination with perturbation theory to correct the wave function (MRD-CI/B-K) employi ng extended atomic orbital (AO) basis sets yielded very accurate hyper fine properties. The theoretical values for CCO are in excellent agree ment with the experimental values determined by Smith and Weltner [J. Chem. Phys. 62, 4592 (1975)]. For CNN, the first assignment of Smith a nd Weltner for the two nitrogen atoms has to be changed. A qualitative discussion of the electronic structure discloses no simple relation b etween the structure of the singly occupied orbitals and the measured hyperfine coupling constants. Vibrational effects were found to be of little importance.